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Moon and Companions

StarDate:

August 21, 2012

There’s quite the little traffic jam in the western sky this evening — a conjunction of the Moon, two planets, and a bright star. The entire group becomes visible as twilight fades from the sky, about 30 to 45 minutes after sunset, and drops from sight a couple of hours later.

Spica, the brightest star of Virgo, is the closest of the other three to the Moon. It’s just to the upper right of the Moon, with the planet Saturn above it. The other planet, Mars, is to the upper left of the Moon.

The Moon is a pretty thin crescent — sunlight illuminates only about 20 percent of the lunar hemisphere that faces our way. As the sky darkens, though, the rest of the lunar disk is clearly visible — lit up by sunlight reflecting off of Earth. It gives the Moon a ghostly glow.

In fact, if you were standing on the Moon and looking back this way, you’d see a big gibbous Earth in the sky. As the hours rolled by, you’d see Earth turning on its axis, with new continents and oceans rotating into view. And as the days rolled by, you’d see the illuminated fraction of Earth grow smaller and smaller.

What you wouldn’t see is Earth moving across the sky. Because the same side of the Moon always faces Earth, from any given point on that hemisphere, Earth would remain at the same place in the sky, so you wouldn’t see Earth rise and set. For that, you’d have to travel around to the lunar farside, and watch Earth set as you moved out of view.

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